


fixation

by kinkykawa



Series: youngblood (miyacest one-shots) [6]
Category: Haikyuu!!
Genre: Age Difference, Age Regression/De-Aging, Anal Sex, Barebacking, Bottom Miya Atsumu, Creampie, Cunnilingus, De-Aged Miya Atsumu, M/M, Multiple Orgasms, Paraphilias, Timeskip Osamu x High School Atsumu, Top Miya Osamu, Trans Male Character, Trans Miya Atsumu
Language: English
Status: Completed
Published: 2020-09-21
Updated: 2020-09-21
Packaged: 2021-03-08 03:27:04
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 1,981
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26579053
Author URL: https://archiveofourown.org/users/kinkykawa/pseuds/kinkykawa
Summary: Osamu's movements falter as he realizes — that’s what Atsumu knows, currently. Not the knowing, expert way Osamu can take him apart at twenty-three, but the clumsy exploration of an inexperienced teenage boy.Well no wonder Atsumu’s hot for it. Osamu is, now, too.(or: atsumu gets magically de-aged and osamu fucks him. that's it, that's the fic.)
Relationships: Miya Atsumu/Miya Osamu
Series: youngblood (miyacest one-shots) [6]
Series URL: https://archiveofourown.org/series/1933474
Comments: 12
Kudos: 441





	fixation

**Author's Note:**

  * For [DalHnrz](https://archiveofourown.org/users/DalHnrz/gifts).



> written as another request for D!! the request was for timeskip osamu fucking de-aged atsumu. no worries, in this fic, we don't lose that sweet twincest flavor LMAO
> 
> as usual, i use mostly neutral/non-explicit terms when referring to atsumu's body, but keep in mind he's FTM trans before reading. author is queer.
> 
> enjoy the fic let us all join hands and walk backwards from god out of spite.

* * *

  
Osamu doesn’t know how he’d forgotten how much of a _brat_ Atsumu had been at seventeen.

Granted, he hadn’t been much better — just much subtler about it. But Atsumu at seventeen was decidedly more willful and selfish, more headstrong about his whims. More defiant. And certainly more precocious and mischievous, untempered by the experience of growing up and realizing you’re not the protagonist of every story.

But there was also something about that catty, selfish, spoilt boy that had kept Osamu reeled in. That he’d been attracted to. That he’d wanted and adored. Osamu knows that even now; Atsumu at seventeen knew it then. Knows that still.

Which brings Osamu to his current dilemma.

Inexplicably, against the rules of reality, Atsumu had woken up today as his seventeen-year-old self. Messy hair, four centimeters shorter, with that little bit of baby fat still in his cheeks. Frustratingly, carelessly beautiful.

“So yer tellin’ me,” he drawls, sprawled out on their couch, kicking his legs in the air, “I’m a pro now?”

Osamu sighs from where he stands in their kitchen, trying to prepare for lunch. “Yes,” he replies, chopping the garlic with a little too much force.

“And Shoyo and Koutarou and Omi-kun are my teammates?”

“Unfortunately for them.”

“ _Hey._ ” Atsumu sits up, glowering at Osamu over the back of the couch. “I bet I’m great fucking setter now.”

“Ya served out of bounds three times last week.”

Atsumu huffs at that, flopping back down onto the cushions with a derisive noise. Some things, Osamu thinks, don’t change. He scrapes the minced garlic into the bowl where the beef is marinating, tossing it all together. Then he sets that aside, washes his hands, and heads into the living room.

“Move,” he mutters, shoving at Atsumu’s legs until his twin relents and allows him space on the couch. Osamu drops down, head leaned back and legs splayed in front of him. He’s exhausted and it’s not even noon. He’d even had to keep the shop closed for the day to deal with… _this._ But he didn’t want to leave Atsumu alone, not in an unfamiliar house in a new city, without his memories of the last six years. Even he isn’t that big of a dick.

Atsumu curls up on the opposite end of the couch, eyeing him carefully. “Osaka, huh?” he asks.

“Mm.” Osamu closes his eyes. “Best offer ya got.”

“Huh.” A pause. “And we’re living together?”

“Much to my regret.”

A scoff, then a longer pause. When Osamu opens his eyes, tilts his head to look at his twin, he finds Atsumu watching him intently.

“Ya really ain’t playin’ anymore.” A statement, not a question. Careful and inflectionless, but Osamu can hear the accusation underneath. He almost smiles.

“Nah.” A corner of his mouth quirks. Atsumu as a teenager, so selfish and demanding. “I told ya, didn’t I? High school was it.”

(At twenty-three, they’ve both made their peace.)

“We still fucking?”

The non-sequitur stalls the breath in Osamu’s lungs. It — honestly, he wasn’t sure if Atsumu would want to address that, no matter that he’d woken up in what was clearly Osamu’s bed, half the mattress vacated but still warm. There’s a difference between sleeping together and _sleeping together;_ Osamu might have still passed that off as Atsumu creeping into his bed, wanting company on a difficult night. But Atsumu at seventeen can still pin him with a look that cuts down to bone, so Osamu cannot lie.

“Yeah,” he murmurs, swallowing. “We are.”

Atsumu unfolds himself a little, all foxkill gaze.

“So why aren’tcha touching me?”

His twin sits up, shifting to hands and knees, and every inch closer he gets makes it harder for Osamu to breathe. He feels caught out, exposed, raw. “You’re _seventeen,_ ” he rasps, his best excuse for keeping his hands to himself.

Atsumu throws a leg over both of his, straddling his lap. Stubborn, selfish brat.

“We’re _twins,_ ” he says, tugging on Osamu’s hair, “and my _age_ is the problem here?”

Osamu growls, all gritted teeth and thinly-veiled frustration. He reaches up, dragging Atsumu down for a harsh, open-mouthed kiss. It’s messy, what with the way Atsumu grins smugly, winding his arms around Osamu’s neck and grinding his hips down.

“Been distracted by yer cock all morning,” he whispers, breathless, cheeky. “Ya wear sweatpants while commando and expect me not to want that in me?”

“Shut up,” Osamu retorts, and then follows up with another kiss.

It’s easy to get lost in this — the press of their lips, for all that Atsumu feels a little unfamiliar, not at all like the last time they’d fucked. Slightly smaller; a little more slender, without the bulk and muscle that adulthood had brought. Graceless and eager, rutting against Osamu, all wandering hands and greedy mouth and—

“Is this seriously turning you on,” Osamu demands, pulling away to stare at Atsumu in disbelief.

“Got a problem with that?” Atsumu counters, rocking his hips down again, and any retort Osamu might have is lost as his twin gets up just long enough to kick his underwear off. He hauls Atsumu back into his lap, hand slipping between his legs to find him already wet, slick all over Osamu’s fingers.

“Ya got the worst fuckin’ kinks,” Osamu mutters, turning his head to bite at Atsumu’s throat.

“Less talking, more fucking,” Atsumu gasps, reaching down to urge Osamu’s fingers inside him.

It’s — smaller, just a bit; tighter. Osamu still remembers being sixteen and falling into bed with Atsumu for the first time, finally daring to cross that last boundary. Awkward and fumbling and immature. His movements falter as he realizes — that’s what Atsumu knows, currently. Not the knowing, expert way Osamu can take him apart at twenty-three, but the clumsy exploration of an inexperienced teenage boy.

Well no wonder Atsumu’s hot for it. Osamu is, now, too.

He yanks his hand away — Atsumu makes an irritated noise — but Osamu just flips them over, pinning Atsumu to the couch. He parts his twin’s legs, curling a hand around one ankle and dragging an open mouth over skin. There’s a flush on Atsumu’s cheeks, trailing down to his chest and deepening the further up Osamu goes. The bravado is gone now, replaced with a shyness kept hidden from everyone else — except here, like this, when it’s just the two of them.

Osamu bites down on the taut muscle of one thigh, making Atsumu cry out. He sucks a bruise in, then another just above it. Trails little nips and licks until he can drag his tongue over the crease at Atsumu’s hip, tasting salt-sweat.

“‘Samu,” his twin exhales shakily. And how can Osamu resist?

The first touch of his mouth to slick folds has Atsumu’s hips jerking, thighs threatening to close before Osamu pushes against one leg. He keeps his eyes open, fixed on Atsumu’s face, watching every reaction — every gasp, every sharp inhale, every press of teeth to a bitten-red lip. He swipes his tongue over, sucks at a sensitive clit, eating Atsumu out like a man starved. Until Atsumu’s shaking, hand clamped over his mouth to muffle his voice, so pretty with tears at the corners of his eyes.

When Osamu teases two fingers inside, curling and fucking them into Atsumu with vicious precision—

He doesn’t think he’ll ever tire of the way Atsumu cries his name when he comes.

Osamu pushes up, swiping the back of his hand over his mouth, licking sweet-salt slick off his palm. “There ya go,” he murmurs. When Atsumu kicks at him vindictively, weakly, he laughs. “Easy, kitten,” smug and satisfied, tugging his twin’s legs around him. “Got what ya wanted?”

“I want ya ta _fuck me,_ ” Atsumu says, pettily, imperious despite the tremor in his voice. So childish, so greedy.

Fuck, Osamu adores him.

He shoves down his sweatpants, freeing his cock with a hiss. Atsumu’s eyes go wide, then one pointed tooth snags his bottom lip in a grin. “He- _llo,_ ” his twin drawls, parting his thighs wider, reaching a hand down to frame his entrance with two fingers. “S’ that for me?”

It shouldn’t be this hot, watching his de-aged twin’s inept attempt at seduction, but god if it doesn’t get Osamu harder anyway.

“Condom,” he bites out, glancing around — he doesn’t remember where he’d stashed them last time. But Atsumu’s leg hooks around his hip and drags him in, making him brace himself against the back of the couch to avoid unbalancing.

“Don’t want one,” his twin tells him blithely. “Wanna feel ya.”

(He shouldn’t, he should insist, should be responsible since he’s older. But Atsumu’s expression is insistent, haughty, and Osamu—

Osamu is more turned on by this than is appropriate, and he doesn’t care.)

He leans down, mouth on Atsumu’s chest, littering bite marks up to a slender throat. He _feels_ Atsumu’s strangled moan as he eases his cock in, inch by deliberate inch. He doesn’t relent until their hips are flush, until he’s buried inside that tight heat.

“Like that?” he quips, breathless, and then without waiting for an answer, Osamu pulls back and thrusts.

He swallows Atsumu’s shout between his teeth.

Every drag of his cock in and out of Atsumu is bliss. He moves — not slow, but intent, rolling his hips and sinking in deep, alternating with shallow grinds that have Atsumu gasping. He pushes two fingers into Atsumu’s mouth just to watch him gag for it, imagining how that inexperienced mouth would feel around his cock. Maybe Atsumu will let Osamu fuck his throat later, will let Osamu re-teach him how to handle a cock between stretched-out lips. He brushes away the tears gathering in Atsumu’s eyes, so pretty, had Atsumu really looked this stunning at seventeen, all messed up and debauched? So young and naive, whining from a cock in him and three fingers pressed to his tongue.

“So good, ‘Tsumu,” he says, caging his twin underneath him, dropping more of his weight down to deepen his thrusts. “Always so good for me, aren’t ya — fuck, so _tight_ — gonna fill you up, take me so well—”

“Do it,” Atsumu gasps, voice raw. His nails draw stinging lines over Osamu’s shoulders, his back. He looks such a _wreck_ and Osamu feels drunk on it. “Fuck, ‘Samu — ‘m gonna—”

He throws his head back as he climaxes a second time, clenching around Osamu’s cock with every shudder that rips through his body. Osamu groans, hips stuttering as he chases his own release. When he comes, it’s with teeth pressed to Atsumu’s shoulder, his twin’s name sweet on his tongue. He pulls out before he’s gone soft, plunging three fingers into Atsumu’s dripping folds and fucking his cum back inside until Atsumu’s sobbing through a third orgasm, pleading for Osamu to stop.

The apartment is quiet but for the sound of their breathing, loud and heavy. Osamu slumps sideways, lifting a hand to lazily lick slick and cum off his skin. He’s caught off-guard when Atsumu heaves himself up on shaky limbs, gracelessly kissing the taste of them out of Osamu’s mouth. It takes a few fumbling moments before Osamu can rearrange them on the cushions so they don’t topple off, Atsumu back in his lap with Osamu leaning against the arm of the couch. He’s never considered himself having an age kink, but this version of Atsumu is rapidly convincing him otherwise.

“D’ya think it’ll wear off soon?” he asks drowsily, trailing his fingers up and down Atsumu’s thigh. His twin shrugs, careless, idling one finger over Osamu’s chest.

“Don’t even know how it happened,” he points out. His hair is a mess. Osamu reaches up, pushing sweat-matted bangs up to better see his twin’s expression.

“Well,” he muses, “no reason we can’t enjoy ourselves until yer back to normal.”

The smirk on Atsumu’s face is the same at seventeen and twenty-three.

“No reason at all.”

**Author's Note:**

> COOL COOL COOL i hope you guys liked that. feel free to leave a comment on anon/guest, or find me on twitter as [@kinkykawa_](https://twitter.com/kinkykawa_) for more kinky content. read the carrd before following!!


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